Without limiting the scope of the invention, its background is described in connection with TAB mounted semiconductor microchips, as an example.
Semiconductors are widely used in integrated circuits for electronic applications, including high speed computers and wireless communications. Such integrated circuits typically use multiple transistors fabricated in single crystal silicon. Many integrated circuits now contain multiple levels of metallization for interconnections. A single semiconductor microchip may have thousands, and even millions of transistors. Logically, a single microchip may also have millions of lines interconnecting the transistors. In addition, modem microchips have numerous other elements that make up part of the integrated circuit. As electrical current runs through these elements at astronomical speeds, heat dissipation becomes a major concern. Therefore, thermal relief becomes a major design requirement for modem microchips. In addition to the thermal relief that modern microchips require, some of these microchips also require an electrical contact as the chip is mounted to the circuit board.
In addition, increased miniaturization in the electronics industry is causing more and more components to be placed onto circuit boards by the Tape Automated Bonding (TAB) method Securing the leads on the device to the board can be achieved by two processes--"Formed Lead" and "No Form" process. Each has its advantages and disadvantages.
In the Formed Lead process, leads are excised and formed to a gull-wing shape. In this process, the leads are formed in such a way that the die-attach thickness is accommodated between the die and the printed wiring board. In this process, the consistency of the die-attach is very critical. Warpage of the board can cause delamination or unsatisfactory coverage.
In the No Form process, leads are excised but not formed Leads have and act much like angular cantilever beams. However, die-attach thickness is less critical in this method since the cantilever shaped leads are bent down during the bonding process. Accordingly, die-attach thickness may vary in this process. Not surprisingly, industry is slowly switching over to the No Form lead process. One of the driving forces for the switch-over is the ease of machine vision rather than the ease of die-attach.